The present invention relates to continuous casting of aluminum alloys, more particularly, to continuous casting aluminum alloys between two cooled rolls at speeds of over 25 feet per minute
Continuous casting of metals such as aluminum alloys is performed in twin roll casters, block casters and belt casters. Twin roll casting of aluminum alloys has enjoyed good success and commercial application despite the relatively low production rates achievable to date. The present invention is directed to a method of continuous casting aluminum which surpasses the productivity of twin roll casting and reaches a level comparable to or better than the productivity of belt casting.
Twin roll casting traditionally is a combined solidification and deformation technique involving feeding molten metal into the bite between a pair of counter-rotating cooled rolls wherein solidification is initiated when the molten metal contacts the rolls. Solidified metal forms as a xe2x80x9cfreeze frontxe2x80x9d of the molten metal within the roll bite and solid metal advances towards the nip, the point of minimum clearance between the rolls. The solid metal passes through the nip as a solid sheet. The solid sheet is deformed by the rolls (hot rolled) and exits the rolls.
Aluminum alloys have successfully been roll cast into xc2xc inch thick sheet at about 4-6 feet per minute or about 50-70 pounds per hour per inch of cast width (lbs/hr/in). Attempts to increase the speed of roll casting typically fail due to centerline segregation. Although it is generally accepted that reduced gauge sheet (e.g. less than about xc2xc inch thick) potentially could be produced more quickly than higher gauge sheet in a roll caster, the ability to roll cast aluminum at rates significantly above about 70 lbs/hr/in has been elusive.
Typical operation of a twin roll caster at thin gauges is described in U.S. Pat. No. 5,518,064 (incorporated herein by reference) and depicted in FIGS. 1 and 2. A molten metal holding chamber H is connected to a feed tip T which distributes molten metal M between water-cooled twin rolls R1 and R2 rotating in the direction of the arrows A1 and A2, respectively. The rolls R1 and R2 have respective smooth surfaces U1 and U2; any roughness thereon is an artifact of the roll grinding technique employed during their manufacture. The centerlines of the rolls R1 and R2 are in a vertical or generally vertical plane L (e.g. up to about 15xc2x0 from vertical) such that the cast strip S forms in a generally horizontal path. Other versions of this method produce strip in a vertically upward direction. The width of the cast strip S is determined by the width of the tip T. The plane L passes through a region of minimum clearance between the rolls R1 and R2 referred to as the roll nip N. A solidification region exists between the solid cast strip S and the molten metal M and includes a mixed liquid-solid phase region X. A freeze front F is defined between the region X and the cast strip S as a line of complete solidification.
In conventional roll casting, the heat of the molten metal M is transferred to the rolls R1 and R2 such that the location of the freeze front F is maintained upstream of the nip N. In this manner, the molten metal M solidifies at a thickness greater than the dimension of the nip N. The solid cast strip S is deformed by the rolls R1 and R2 to achieve the final strip thickness. Hot rolling of the solidified strip between the rolls R1 and R2 according to conventional roll casting produces unique properties in the strip characteristic of roll cast aluminum alloy strip. In particular, a central zone through the thickness of the strip becomes enriched in eutectic forming elements (eutectic formers) in the alloy such as Fe, Si, Ni, Zn and the like and depleted in peritectic forming elements (Ti, Cr, V and Zr). This enrichment of eutectic formers (i.e. alloying elements other than Ti, Cr, V and Zr) in the central zone occurs because that portion of the strip S corresponds to a region of the freeze front F where solidification occurs last and is known as xe2x80x9ccenterline segregationxe2x80x9d. Extensive centerline segregation in the as-cast strip is a factor that restricts the speed of conventional roll casters. The as-cast strip also shows signs of working by the rolls. Grains which form during solidification of the metal upstream of the nip become flattened by the rolls. Therefore, roll cast aluminum includes grains with multiaxial (non-equiaxed) structure.
The roll gap at the nip N may be reduced in order to produce thinner gauge strip S. However, as the roll gap is reduced, the roll separating force generated by the solid metal between the rolls R1 and R2 increases. The amount of roll separating force is affected by the location of the freeze front F in relation to the roll nip N. As the roll gap is reduced, the percentage reduction of the metal sheet is increased, and the roll separating force increases. At some point, the relative positions of the rolls R1 and R2 to achieve the desired roll gap cannot overcome the roll separating force, and the minimum gauge thickness has been reached for that position of the freeze front F.
The roll separating force may be reduced by increasing the speed of the rolls in order to move the freeze front F downstream towards the nip N. When the freeze front is moved downstream (towards the nip N), the roll gap may be reduced. This movement of the freeze front F decreases the ratio between the thickness of the strip at the initial point of solidification and the roll gap at the nip N, thus decreasing the roll separating force as proportionally less solidified metal is being compressed and hot rolled. In this manner, as the position of the freeze front F moves towards the nip N, a proportionally greater amount of metal is solidified and then hot rolled at thinner gauges. According to conventional practice, roll casting of thin gauge strip is accomplished by first roll casting a relatively high gauge strip, decreasing the gauge until a maximum roll separating force is reached, advancing the freeze front to lower the roll separating force (by increasing the roll speed) and further decreasing the gauge until the maximum roll separating force is again reached, and repeating the process of advancing the freeze front and decreasing the gauge in an iterative manner until the desired thin gauge is achieved. For example, a 10 millimeter strip S may be rolled and the thickness may be reduced until the roll separating force becomes excessive (e.g. at 6 millimeters) necessitating a roll speed increase.
This process of increasing the roll speed can only be practiced until the freeze front F reaches a predetermined downstream position. Conventional practice dictates that the freeze front F not progress forward into the roll nip N to ensure that solid strip is rolled at the nip N. It has been generally accepted that rolling of a solid strip at the nip N is needed to prevent failure of the cast metal strip S being hot rolled and to provide sufficient tensile strength in the exiting strip S to withstand the pulling force of a downstream winder, pinch rolls or the like. Consequently, the roll separating force of a conventionally operated twin roll caster in which a solid strip of aluminum alloy is hot rolled at the nip N is on the order of several tons per inch of width. Although some reduction in gauge is possible, operation at such high roll separating forces to ensure deformation of the strip at the nip N makes further reduction of the strip gauge very difficult. The speed of a roll caster is restricted by the need to maintain the freeze front F upstream of the nip N and prevent centerline segregation. Hence, the roll casting speed for aluminum alloys has been relatively low.
Some reduction in roll separating force to obtain acceptable microstructure in alloys having high alloying element content is described in U.S. Pat. No. 6,193,818. Alloys having 0.5 to 13 wt. % Si are roll cast into strip about 0.05 to 0.2 inch thick at roll separating forces of about 5000 to 40,000 lbs/in at speeds of about 5 to 9 ft/min. While this represents an advance in roll separating force reduction, these forces still pose significant process challenges. Moreover, the productivity remains compromised and strip produced according to the ""818 patent apparently exhibits some centerline segregation and grain elongation as shown in FIG. 3 thereof.
A major impediment to high-speed roll casting is the difficulty in achieving uniform heat transfer from the molten metal to the smooth surfaces U1 and U2. In actuality, the surfaces U1 and U2 include various imperfections which alter the heat transfer properties of the rolls. At high rolling speeds, such nonuniformity in heat transfer becomes problematic. For example, areas of the surfaces U1 and U2 with proper heat transfer will cool the molten metal M at the desired location upstream of the nip N whereas areas with insufficient heat transfer properties will allow a portion of the molten metal to advance beyond the desired location and create nonuniformity in the cast strip.
Thin gauge steel strip has been successfully roll cast in vertical casters at high speeds (up to about 400 feet/min) and low roll separating forces. The rolls of a vertical roll caster are positioned side by side so that the strip forms in a downward direction. In this vertical orientation, molten steel is delivered to the bite between the rolls to form a pool of molten steel. The upper surface of the pool of molten steel is often protected from the atmosphere by means of an inert gas. While vertical twin roll casting from a pool of molten metal is successful for steel, aluminum alloys cannot be cast from a pool of molten aluminum alloy. The molten aluminum in such a pool at the bite of vertical rolls would readily oxidize even when protected. This would change the metallurgical properties of the alloy being cast. Steel alloys are much less susceptible to oxidation problems, and with proper protection from oxidation, can be successfully roll cast.
One suggestion for overcoming this problem of oxidized aluminum in vertical roll casting on a laboratory scale is described in Haga et al., xe2x80x9cHigh Speed Roll Caster for Aluminum Alloy Stripxe2x80x9d, Proceedings of ICAA-6, Aluminum Alloys, Vol. 1, pp. 327-332 (1988). According to that method, a stream of molten aluminum alloy is ejected from a gas-pressurized nozzle directly onto one or both of the twin rolls in a vertical roll caster. Although high speed casting of aluminum alloy strip is reported, a major drawback to this technique is that the delivery rate of the molten aluminum alloy must be carefully controlled to ensure uniformity in the cast strip. When a single stream is ejected onto a roll, that stream is solidified into the strip. If a stream is ejected onto each roll, each stream becomes one half of the thickness of the cast strip. In both cases, any variation in the gas pressure or delivery rate of the molten aluminum alloy results in nonuniformity in the cast strip. The control parameters for this type of aluminum alloy roll casting are not practical on a commercial scale.
Continuous casting of aluminum alloys has been achieved on belt casters at rates of about 20-25 feet per minute at about xc2xe inch (19 mm) gauge reaching a productivity level of about 1400 pounds per hour per inch of width. In conventional belt casting as described in U.S. Pat. No. 4,002,197, molten metal is fed into a casting region between opposed portions of a pair of revolving flexible metal belts. Each of the two flexible casting belts revolves in a path defined by upstream rollers located at one end of the casting region and downstream rollers located at the other end of the casting region. In this manner, the casting belts converge directly opposite each other around the upstream rollers to form an entrance to the casting region in the nip between the upstream rollers. The molten metal is fed directly into the nip. The molten metal is confined between the moving belts and is solidified as it is carried along. Heat liberated by the solidifying metal is withdrawn through the portions of the two belts which are adjacent to the metal being cast. This heat is withdrawn by cooling the reverse surfaces of the belts by means of rapidly moving substantially continuous films of water flowing against and communicating with these reverse surfaces.
The operating parameters for belt casting are significantly different from those for roll casting. In particular, there is no intentional hot rolling of the strip. Solidification of the metal is completed in a distance of about 12-15 inches (30-38 mm) downstream of the nip for a thickness of xc2xe inch. The belts are exposed to high temperatures when contacted by molten metal on one surface and are cooled by water on the inner surface. This may lead to distortion of the belts. The tension in the belt must be adjusted to account for expansion or contraction of the belt due to temperature fluctuations in order to achieve consistent surface quality of the strip. Casting of aluminum alloys on a belt caster has been used to date mainly for products having minimal surface quality requirements or for products which are subsequently painted.
The problem of thermal instability of the belts is avoided in block casters. Block casters include a plurality of chilling blocks mounted adjacent to each other on a pair of opposing tracks. Each set of chilling blocks rotates in the opposite direction to form a casting region therebetween into which molten metal is delivered. The chilling blocks act as heat sinks as the heat of the molten metal transfers thereto. Solidification of the metal is complete about 12-15 inches downstream of the entrance to the casting region at a thickness of xc2xe inch. The heat transferred to the chilling blocks is removed during the return loop. Unlike belts, the chilling blocks are not functionally distorted by the heat transfer. However, block casters require precise dimensional control to prevent gaps between the blocks which cause nonuniformity and defects in the cast strip.
This concept of transferring the heat of the molten metal to a casting surface has been employed in certain modified belt casters as described in U.S. Pat. Nos. 5,515,908 and 5,564,491. In a heat sink belt caster, molten metal is delivered to the belts (the casting surface) upstream of the nip with solidification initiating prior to the nip and continued heat transfer from the metal to the belts downstream of the nip. In this system, molten metal is supplied to the belts along the curve of the upstream rollers so that the metal is substantially solidified by the time it reaches the nip between the upstream rollers. The heat of the molten metal and the cast strip is transferred to the belts within the casting region (including downstream of the nip). The heat is then removed from the belts while the belts are out of contact with either of the molten metal or the cast strip. In this manner, the portions of the belts within the casting region (in contact with the molten metal and cast strip) are not subjected to large variations in temperature as occurs in conventional belt casters. The thickness of the strip can be limited by the heat capacity of the belts between which casting takes place. Production rates of 2400 lbs/hr/in for 0.08-0.1 inch (2-2.5 mm) strip have been achieved.
However, problems associated with the belts used in conventional belt casting remain. In particular, uniformity of the cast strip depends on the stability of (i.e. tension in) the belts. For any belt caster, conventional or heat sink type, contact of hot molten metal with the belts and the heat transfer from the solidifying metal to the belts creates instability in the belts. Further, belts need to be changed at regular intervals which disrupts production.
Accordingly, a need remains for a method of high-speed continuous casting of aluminum alloys without using a pair of belts and which achieves uniformity in the cast strip surface.
This need is met by the method of the present invention of continuous casting aluminum alloy which includes delivering molten aluminum alloy juxtaposed and in communication with a pair of water-cooled rolls arranged in a generally horizontal plane. A reservoir of molten aluminum alloy is advanced towards a nip between the rolls. Outer layers of solid aluminum alloy results on each of the rolls, and a semi-solid aluminum layer is produced in the center between the solid layers. The semi-solid layer includes a molten component and a solid component of broken dendritic arms detached from the solidification front. The solid outer layers and the solid component of the semi-solid aluminum alloy pass through the nip such that a strip of solid aluminum alloy exits the nip while the molten component of the aluminum alloy is urged upstream from the nip. The strip exiting the nip includes a solid central segregated layer sandwiched between the outer conforming solid layers of aluminum alloy. Under typical conditions, the thickness of the center layer is about 20 to about 30% of the total strip thickness. In this manner, a solid strip of aluminum alloy is not produced until the alloy reaches the forming point of the nip. Moreover, unlike in conventional twin roll casters, the rolls do not substantially deform the strip of cast aluminum, a result of which is that the process operates at very low roll separating force.
The molten aluminum alloy has an initial concentration of eutectic forming alloying elements. A result of producing the segregated portion from the broken dendritic arms of the alloy is that this segregated portion is depleted of the eutectic forming alloying elements. The concentration of the eutectic forming alloying elements in the intermediate layer is less than the concentration of the eutectic forming alloying elements in each of the outer layers by as much as about 5 to about 20%.
The strip of metal may exit the nip at a rate of about 25 to about 400 feet (7.7-123 m) per minute or at a rate of about 100 to about 300 feet (30-92 m) per minute. The linear speed at which the solid strip is produced is higher than the linear rate at which the molten aluminum alloy is delivered to the rolls, such as about four times higher than the linear rate of the molten aluminum alloy. The rolls are arranged to cast the strip in a generally horizontal configuration and may be textured with surface irregularities (e.g. grooves, dimples or knurls) about 5 to about 50 microns high and spaced at about 20 to about 120 per inch to enhance heat transfer. The roll separating force is less than about 25 to about 300 pounds per inch of width and may be about 25 to about 200 pounds per inch of width or about 100 pounds per inch of width. The solid strip may be produced in thicknesses of about 0.07 to about 0.25 inch or about 0.08 to about 0.095 inch. The rolls are internally cooled and the contacting surfaces may be oxidized prior to use to provide a continuous and uniform oxide layer thereon. The rolls are brushed periodically or continuously to remove debris that may be deposited during casting. Fixed edge dams and electromagnetic dams may be used to prevent leaking of the molten metal from the sides.